The enormous number of Plasmodium falciparum infections in the world act to generate a vast reservoir of genetic variants, which can influence the pathogenesis, virulence, and drug sensitivity of these parasites. It is generally believed that P. Falciparum isolates are panmixic, i.e., mating randomly in the population. Recent work, however, indicates a pattern of nonrandom mating in some parts of the world. Furthermore, an investigation of maternally inherited cytoplasmic markers in a laboratory cross of well-characterized P. Falciparum clones HB3 and 3D7 revealed unidirectional mating in which the 3D7 clone dominated as the maternal parent. The prevalence of such mating behavior among various geographically distant P. Falciparum strains is unknown at present. Instances of unidirectional cytoplasmic incompatibility are known in many different species, and are often mediated by elements that cause genetic instability and hybrid dysgenesis in unidirectional manner. This often portends early stages of speciation. Hence, an understanding of mating compatibility among various P. Falciparum strains is important in the pursuit of knowledge regarding the emerging and reemerging global malaria situation. The hypothesis under which this project will operate states that P. Falciparum strains are undergoing nonrandom mating, and that unidirectional incompatibility affects parasite mating behavior. It is further hypothesized that a regulated genetic instability is an underlying factor that drives the incompatible mating behavior. Nonrandom mating could have a major impact on the spread of multi- locus traits such as the responses to immune surveillance and drug treatment within the parasite population. Three specific aims involving relatively straightforward approaches, but requiring labor intensive work, are proposed. In aim 1, cross-mating between well characterized P. Falciparum clones from Central America (HB3), Africa (3D7), Southeast Asia (W2), and South America (7G8) will be carried out, and the hybrid oocysts examined for their maternal lineage and directionality of the mating. In aim 2, independent recombinant progeny of the HB3 X 3D7 cross will be backcrossed to the HB3 parent to examine the possible nuclear locus that may dictate unidirectional mating. In aim 3, a genetic linkage map of the HB3 X 3D7 map will be derived using simple sequence length polymorphism markers. The map will aid in positional identification of the genes responsible for important biological properties of P. falciparum.